Web Api Action Filters Do Not Run

FluentValidation: 9.5.0 ASP.NET: 5.0

When I have my own action filters in a Web Api controller, if I am using FluentValidation, the filters do not execute.

Given the following filter:

public class MyActionFilter : IActionFilter
{
	public MyActionFilter()
	{
		System.Diagnostics.Trace.WriteLine( "ctor" );
	}

	public void OnActionExecuting( ActionExecutingContext context )
	{
		System.Diagnostics.Trace.WriteLine( "OnActionExecuting" );
	}

	public void OnActionExecuted( ActionExecutedContext context ) 
	{
		System.Diagnostics.Trace.WriteLine( "OnActionExecuted" );
	}
}

If I configure my builder as the following:

builder
	.AddControllers()
	.AddMvcOptions( opt =>
	{
		opt.Filters.Add( typeof( MyActionFilter ) );
	} )
	.AddFluentValidation( cfg =>
	{
		cfg.RegisterValidatorsFromAssemblyContaining<TValidatorsAssembly>();
	} );

The following occurs:

1. The MyActionFilter.ctor is successfully called.
2. Neither of the OnAction* methods are called.
3. The web api result is a ValidationProblemDetails with correct FluentValidation error messages returned.

If I remove the call to AddFluentValidation, then I do successfully hit the OnAction* methods.

If I try this same pattern for my Razor Pages using an IPageFilter (still using AddFluentValidation), then the OnPageHandlerExecuting is successfully called.

Is this a bug in FluentValidation or am I misunderstanding how the flow should work for web api actions?

[JeremySkinner]

I’d also suggest reading up on how filters work in ASP.NET: https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/filters?view=aspnetcore-5.0 The section on “Cancellation and short-circuiting” explains how one filter can be used to cancel the execution of all others (which is what the ModelStateInvalidFilter is doing - which is enabled when you use ApiController)

[JeremySkinner]

Thought that was a requirement in creating Web Apis

No, it isn’t required. It’s a shortcut that enables a certain set of conventions.

Is there a work around so that filters continue to execute?

You’d have to disable the automatic 400 responses and switch to returning them manually (or use a different filter type). The ASP.NET documentation explains how to do this. https://docs.microsoft.com/en-us/aspnet/core/web-api/?view=aspnetcore-5.0#disable-automatic-400-response I’d also suggest reading the rest of that page, as it explains exactly what the ApiController attribute does.

I’m going to close the issue now as it isn’t related to FluentValidation.